The present invention relates to a polymer inhibiting and polymer viscosity reducing composition and to a process for inhibiting the polymerization of readily polymerizable vinyl aromatic compounds and reducing the viscosity of polymers formed, during heating.
It is well known that vinyl aromatic compounds such as monomeric styrene, alpha-methyl styrene, and the like, polymerize readily and furthermore, that the rate of polymerization increases with increasing temperature. Inasmuch as vinyl aromatic compounds produced by common industrial methods contain impurities, these compounds must be subjected to separation and purification processes in order to be suitable for most types of further industrial use. Such separation and purification is generally accomplished by distillation techniques.
In order to prevent polymerization during distillation of vinyl aromatic compounds, various types of known polymerization inhibitors have been employed in connection with prior art distillation processes. For example, common inhibitors useful for inhibiting the polymerization of vinyl aromatic compounds under distillation conditions include 4-tert-butylcatechol (TBC) and hydroquinone. Additionally, sulfur has been widely employed as a polymerization inhibitor during the distillation of various vinyl aromatic compounds. However, while sulfur provides a reasonably effective inhibitor, its use in such distillation processes results in a highly significant disadvantage, namely, there is formed in the reboiler bottoms of the distillation column a valueless waste material highly contaminated with sulfur. This waste material furthermore represents a significant problem of pollution and waste removal.
Although many compounds are effective for inhibiting the polymerization of vinyl aromatic compounds under differing conditions, for example storage, only some of these compounds have proved to be any real utility for inhibiting vinyl aromatic polymerization under distillation conditions. One compound found effective for polymerization inhibition is 2,6-dinitro-p-cresol (DNPC) disclosed in U.S. Pat. No. 4,105,506 by Watson, incorporated herein by reference, where DNPC was found to be an effective inhibitor. It has been found, however, as the distillation temperature increases the effectiveness of DNPC decreases. In addition, it has been found that previously known polymerization inhibitors may be combined to achieve an inhibitory effect greater than either inhibitor alone. The synergistic effect of combining two known inhibitors was disclosed in U.S. Pat. No. 4,061,545 by Watson, incorporated herein by reference, wherein phenothiazine and 4-tert-butylcatechol were used together in the presence of oxygen as a polymerization inhibitor. It has been found, however, that when utilizing the phenothiazine/TBC mixture, a certain amount of polymerization of high molecular weight still occurs. This high molecular weight polymer results in a highly viscous polymer tar which decreases heat transfer within the reboiler areas of the distillation train and decreases recovery of the monomer trapped in the polymer tar. In addition, it has been found to be difficult to obtain complete dispersion of air throughout the distillation column thereby further reducing the effectiveness of the oxygen dependent phenothiazine/TBC inhibitor.